Tunnel kilns are elongated kilns through which a train of kiln cars is advanced to heat or fire ceramic materials, such as bricks, supported on the kiln cars. The kiln car train typically travels on rails running through the tunnel. The material to be heated is supported on a flat deck which in turn is supported on an undercarriage with wheels which travel on the rails. It is known to cool the underside of tunnel kiln cars, such that the support and transport mechanisms are maintained in a relatively cooler atmosphere than the upper deck side. Cooling of the underside of the kiln cars is utilized to avoid overheating the undercarriage, wheels, bearings and the like located beneath the deck of the kiln cars.
The kiln tunnel is generally separated into an under-car tunnel area or channel and an above-car tunnel area or channel by the deck of the kiln cars and one or more mechanical seals connected to or associated with the kiln car decks. The seals function to attempt to keep heated and cooled air in their respective areas, such that heated air does not migrate from the above-car channel to the below-car channel and cooling air does not migrate from the under-car channel into the above-care channel and have to be heated to process temperatures. These mechanical seals are specifically necessary to prevent infiltration and ex-filtration of air into and out of the above-car channel as the primary kiln exhaust fan, typically located toward the kiln tunnel entrance, keeps a relatively negative pressure in the above-car channels, which are heated to process temperatures.
One conventional and sometimes additional method for sealing the moving kiln car sides to the kiln side walls is to provide aprons along the longitudinal car sides which dip into sand filled channels of the kiln side walls such that the sand forms a closed barrier extending the length of the kiln. Transverse joints between successive kiln cars may be sealed by means of conventional mechanical joints and elastic material cords. The purpose of such mechanical seals and sand barriers is to substantially prevent pressure equalization between the under-car channel and the heated above-car channels, the seals are far from perfect. For design and cost reasons, the depth to which aprons can dip into the sand must be relatively small. Additionally, the sand must be fairly coarse so that it will be heavy enough so as not to be blown out of the channel barrier area and entrained in the moving gas flows. As a result, the sand barrier actually is permeable to gas and does not provide a perfect seal. Mechanical and elastic material seals simply wear out and degrade from the excessive kiln temperatures and also do not provide a perfect seal.
An established method of cooling the under-car channel is forcing air through the under-car channel at each of the various heating zones in the tunnel kiln. A disadvantage of this method is that a portion of the forced air will penetrate the mechanical seals and then the cooling air will have to be heated to very high process temperatures. A second method of cooling the under-car channel is forcing air into the under-car channel from the exit end of the tunnel kiln toward its entrance end, which may or may not be practiced with a secondary under-car exhaust fan located toward the kiln entrance which draws air from the under-car channel. This second method also has a disadvantage in that a portion of the forced air will penetrate the mechanical seals and then the cooling air will have to be heated to very high process temperatures.
A third method is to use openings in the foundation or side walls of the under-car channel to allow natural cooling of the under-car channels. This third method also has a disadvantage in that a portion of the natural cooling air will penetrate the mechanical seals and then the cooling air will have to be heated to very high process temperatures. The cooling air penetration in all three cases of the known prior art is partially caused by imperfect and worn mechanical seals, misaligned seals caused by natural degradation of the tunnel kiln structure, and the negative pressure within the above-car channel caused by the kiln exhaust fan, i.e. a pressure imbalance between the under-car channel and above-car channel.
The above-car channel is typically filled with air, combustion products, and off gases (collectively gases) from the heating process and curing process from the ceramic materials. These gases are typically flowing the same direction as the under-car cooling air, such that a pressure gradient develops in both channels. Because there are different gas flow rates and resistances in the above-car and under-car channels, the pressure gradient is different as a function of distance along the tunnel thereby leading to “false” air flows between the two channels, usually in the form of air moving from the under-car channel to the above-car channel. The air flows between the two channels (infiltration into the above-car channel and ex-filtration from the under-car channel) must be avoided in order to avoid undue heating of the under-car channel or undue cooling of the above-car channel and the related excess energy usage to heat the infiltrated air from under-car channel.
In prior art it is also known that there may be multiple trains of kiln cars traveling parallel to one another in side-by-side fashion. Such a kiln may or may not have intermediate longitudinal walls located between adjacent kiln car trains. Such kiln cars are typically equipped with the same conventional sand seals, i.e. aprons described above which dip into sand filled channels disposed laterally of each train. The problems associated with this conventional “sand trough” sealing technique for multi-train kilns are simply an order of magnitude larger than those experienced with a single train kiln. For example, long lateral distances are needed between adjacent kiln car trains to accommodate the required volume of sand in the channels in order to seal each of the multiple trains. The long lateral distances and required structure disrupt the gas flow conditions existing in the firing channel. Also, the increased number of sand-sealed channels in multi-train kilns tunnels increases the infiltration into the above-car channels from the under-car channels, making it more difficult to heat the above-car channel and the material on the car. For these reasons, multi-train tunnels are not generally constructed for commercial use.
It would be advantageous if a kiln tunnel could be provided which minimizes “leaks” between the above-car channel and under-car channel while providing balanced under-car cooling. It would be further advantageous to develop such a system which could be utilized with multi-train or multi-track tunnels.